Recently, there has been much interest in the imaging sequence known as fast imaging with steady state precession (FISP). FISP is an imaging sequence that employs balanced steady-state free precession (SSFP), thereby recycling the magnetization, and providing a high signal-to-noise ratio (SNR) that is practically independent of the sequence repetition time TR. As used herein, the term SSFP refers to the general NMR phenomenon of steady-state free precession, and the term FISP refers to an SSFP imaging sequence with fully balanced gradients in each TR.
Fat, especially subcutaneous tissue, generates large signals. These can be a source of artifacts such as Gibbs ringing, and may interfere with the signals of interest by partial volume effects or by obscuring the origin of the signal (e.g., blood). For many practical implementations, the TR of the FISP sequence is such that the contributions of fat in the image are phased-opposed to those of water, leading to destructive interference in pixels with partial volumes of water and fat. Also, methods requiring a reference image (e.g., SENSE) can be corrupted by large fat signals. Fat surrounding the coronary arteries is also detrimental in coronary artery imaging (CAI) since it can obscure the artery or confound signal from contrast-enhanced blood. It is, therefore, often desirable to suppress the contribution of fat signals in the image.
Most common fat attenuation methods rely on the differing evolution of water and fat by, e.g., frequency or relaxation. They thus require significant evolution time to perform selective suppression, and are generally only effective during a limited time-window at a specific delay after application. More recently, methods have been proposed for performing fat suppression or fat/water separations specifically during FISP acquisitions. These methods require phase-cycling or multiple acquisitions, at least doubling the scan time, and are therefore non-ideal for use with real-time imaging. A single acquisition, phase-based postprocessing method has been proposed, but this approach identifies whole voxels as either fat or water and hence will suffer from partial volume effects.
It would be desirable, therefore, if there were available systems and methods that provide significant attenuation of fat-based signal while maintaining the preferred signal level for water-based tissues provided by standard FISP.